Extensometer



Patented Sept. 30, 1952 v ',.EX'rENsoME'rEnL- y l Jack Rebman,` (lolu'mbus,Y Ohio, as sig'norfby I' mesne assignments, to The Battelle Develop` ment Corporation, Columbus, Ohio, a corporation of Delaware Appucation February 9, 1959,;.seriaiNo-1fia244 v y v This invention relates to measuring instruments and more particularly to* a novel extensometer for measuring the degree of elongation of a given material under varying loads, and

which extensometer is iirmly attachedto the specimen in a manner to accurately indicate the degree of elongation thereof, but automatically releases itself from the specimen prior to the time the ultimate tensile strength of the specimen is exceeded and the specimen breaks. r I

Extensometers are used to measure the degree of elongation of specimens during testing in tension and to determine the modulus of elasticity, thev yield point, and other physicalcharacteristics. When tension is applied Auntilthe specimen breaks, there is great danger of injury. to the measuring instrument. In devices of the prior art, it has been necessary to stop the tests and to remove the extensometer from the specimen before the breaking point is reached@ In high-speed testing, this procedure is highlyi impractical. Also, the exact point of rupture can'- not always be predicted.

It is often desirable. to test metal specimens `which have been heatedto a high temperature .1'7

by means of induction or electrical resistance heating. Consequently, it has been found necessary to devise some means to protect the extensometer,l from the high temperature of the test specimen and also to insulate the indicating.` or

recording mechanism from any heat or electrical currents.:V K

Accordingly it is one of the objects of this in'- vention to providean extensometer for usein high-speed tension testing whereby the extening mechanism is insulated from any electrical l current. .e l

A still further object-is to provide means in an extenso'meter for automatically releasing the extensorneter from the test specimen before the breaking point is reached.

Other obiects and advantages will be apparent from the following detailed description and the accompanying drawings wherein similar characters of reference designate corresponding parts, and wherein:

` i liligure irepresents a top view, partially `broken away, of an extensometer embodying rthepresent invention. f

' Figure 2 is a sideview of `theexter`1sor'r'1eter- Figure 3 is partial section taken ,alongv the line 3-3 of Figure 1. Y e

, In general, this novel extensometer comprises two nonflexing arms, each of whichhas one'end adapted to be slightly'imbedded in a test specivmen. Supported on each of these arms is a clip arm which holds the extensometer iirmly tothe specimen. The free ends kof4 the noniiexing varms are'joined by a flexibleplate. Elongation ofthe specimen during testing in tension laterally separates the arms, thereby iiexingthe plate,l and a measurement of this flexing ofthe' platel in'- dicates the amount of elongation' Aof the specimen. A yoke mechanism acts on the clip arms to hold the extensometer inr'osontact` with the specimen, andthen to automatically releaseit'.

A preferred embodiment, as shown in the drawings,ecomprises two nonflexing arms I0 and II, which -terminate in end pieces I2 and I3. Mounted in the en d pieces I2 and I3"arelup' standing-engaging points I4 and 'I4'. v'I'hes'e end pieces and engaging'point's' are Apreferably constructed of some temperature-resistant alloy v such as Inconel-X. The end piecesl I 2 and I3 are rigidly connected tofceramic or'other" insulating sections I5 by means of screws and vbolts or other suitable fastenings I6. e-The insulating l sections I5 are fastened to the main portions of arms I 0 and IIasfimilar-manthe nonflexing ner.

- U-shaped supports v-I'I fastenedV to nonne/:ring arms I 0 and II pivotally.supportlclip'arms yI8 and I9 by means of bolts and nuts', or other s'uitable fastenings'2l. The clip arms l8and" I Binclude insulating sections similar toil'theinsulating sections I5.of the noniiexing arms I0 andL Il Clip-arm hooks f2!) .and 20' positioned directly over the upstanding engaging points f4 andY I4' are rigidly attached tothe clip 'arms.:;-The opposite ends'of'clip arms I8 andv I9 terminate in flat springs 22,. The clip-arm vhooksl) and 2 0' and the sprint-S122 are preferablvprovided with talos for bending and4 seeurement Ato Vthe clip arms I8 andy I9, to avoid weakening, the insulating materialbyvdrilling holes for bolts Aorsimilar fasteningmeans. f y I y yThe nonexing'arms I0 and II-v aremounted in spacedv parallel relationship betweentwo guide plates 23 and 24, as shown in Figure'2 'oflthe drawings. Guide plates zaand 24 are ricidiy'a't'- tached to arm I0 by means of fastenings 25.- Arm H may be slightly thinner than yarm lo or 'the guide plates 23 and 24 may be so spaced from each other that the arm II may move freely laterally about a preselected pivot point or points between guide plates 23 and 24, which are maintained in spaced relation by guide plate spacers 2B, and rigidly held in place by fastenings 21. Such lateral movement of arm 'I I islimited-.by the sloppy to'f the guide plate spacers 26 in oversized holes 2B drilled through the arm,

thereby preventing damage to the extensometer;

due to excessive separation of arms IIJ and II. Cooling holes 29 are drilled in nonexing arms I and II through which air can be forced if additional cooling is desired when high temperatures are used.

preferably made of aluminum or 'some other'material which will give a minimum restraint to the movement of arm I I.

Flexible plate 30 is secured -tovnon-flexing arms I0 and II by retainer plates-3| ,utilizing suitable fastenings 32.

One or more wire strain gages 33 are mounted on the surfaces `of plate 30 .and connected by means-of aWheatstone bridge or. similar circuit to suitable indicating or recording.v mechanism. Sincethe flexing of the `plate Bellis proportional to the-.lateral movement of nonflexing arm Il .about itspivot point the elongation of a test speci-men maybe easily determined. Lead wires .34 maybe passed `through a hole .-35v in Vguide plate 24=tojprotect the connectionl to the strain gage. .Although satisfactory readings may be made by using a single active. strain gage, the use oftwoor more,rpreferably in pairs, will give a l.

.greater sensitivity andwill compensate for temperature-changes. .I

--jAreleasable yoke 36ispivotallymountedon guide'iplate 23 by means of bolt-31 which passes through brackets 38-extending upward from guide plate. 23. VThe-upperend ofgyoke--Si presses against clip-arm springs 22 therebyholding clip arms I8 and l-iirmly against a test specimen. A triggerreturn spring 39 is connected'between the upper end of yoke'361and guide plate 23, and acts to -release Athe pressure-of clip arms I8 and 1I9';jon .'the-itestspecimen by the action of tabs 4D, which'arean `integral'partofyoke 36 and are .bent-aroundtheclip-arrn springs 22. f- .#Theilower -end of yoke 36 is curvedto forma .moves with it. When arm II is moved a suii- .te'sts'pecim'enfYoke 36 is pivoted against the action'of trigger return spring SSVuntil hooklll is-"j'depre'ssed vvsuiiicien'tly to Y permit "the trigger 'pin42 to engage it. While yoke 35 is heldin this depressed position, engaging point 14" on nonflexing arm II is iitted intothe second indentationon specimen 46. This moves trigger pin 42 :so that it engages hooki .retaining it in a depressed position. In this position the upper end b fyokeQSS is forced against clip-arm springs 22,

Viiexiire plate 3G together with the strain gages 33 to bend. As nonflexing arm II moves apart from arm I0 during the testing, trigger pin 42 cient distance, trigger pin 42 will no longer engage hook 4I, and trigger return spring 39 acting on yoke 35'causes it to pivot about its supports 31, relieving the pressure on clip-arm springs 22 and'thereby permitting the extensometer to fall free from test specimen 48.

' A spring 41 and chain 48 may be connected to the extensometer as shown in Figure l. The spring 41 supports the weight of the extensometer during testing so that very small force couples areapplied 'to engaging points I4 and I4. The chain-48= retains the extensometer to the testing machine after 4it Vhas been released from the :specimen inorder to prevent damage to the extensometeras itv falls free.

By varying the length of trigger pin 42 the extensometer may be adjusted to release from test specimen 4S after any particular increase in length ofthe specimen. Trigger pins of varying lengths may be supplied, or the trigger pin may be `made adjustable as to length in any wellknown' manner. The amount of elongation before rupturemay be 'determined by calculation or preliminaryitesti'ng, and the` proper length of trigger :pin can then Vbe selected. 'Although it is customary to test specimens vertically in a testing machine, it can be seen that this extensometer mayv ice-easily attached toa test specimen held in either a: horizontal or vertical position.

can be seen by the preceding description, this extensometeris particularly desirable for high-speed testing. It is simple in construction and does not require particular technical skill foroperation. fIn addition parts may-be 'easily replaced.

It will `be understood that "modicationsmay be made in" the'design and arrangement of the parts, without'departing from the scope "of the invention which is intended to be limited only asrequired by the `following claims. It will be further understood that the size of the apparatus 'may vary, depending upon the size of 'material vwith which itis to be used. 'What' is claimed is:

l. In a substantially U-shaped gagec'omprising a frame andtwo relatively movable specimen-'contacting arms and having means pivotall'y mounted on said arms to hold a specimen in contactwith said arms, a release mechanism comprising a lever pivotally mounted'on said frame, one end of said lever adapted to Contact Asaid'specimen-holding means andl hold Ythem firmly against a specimen, a'hooked member on theV other endv of said lever, means mounted on oneof said arms to slidably engage said hook until said arms have relatively moved a predetermined distance, and resilient means between said framean'd said lever whereby said lever willbe moved from contact withsaid specimen-holding members. y

'ZfA gage comprising a frame, a fixed arm secured to said frameand extending therefrorma movable arm loosely mounted in said frame, said arms and frame forming a .substantially U-shaped unit, means pivotally mounted on each of said arms to hold a specimen in contact with said arms, a lever pivotally mounted on said frame, one end of said lever adapted to contact said specimen-holding means and hold them rmly against the specimen, a hooked member on the other end of Said lever, means mounted on said movable arm to slidably engage said hook until said arm has moved a predetermined distance, and resilient means between said frame and said lever whereby said lever will be moved from contact with said specimen-holding means after said hook is disengaged.

3. A gage comprising a frame, a fixed arm secured to said frame and extending therefrom, a movable armv loosely mounted in said frame, said arms and frame forming a substantially U-shaped unit, specimen-engaging means mounted on the end of each arm, a clip arm pivotally mounted on each of said arms and having one end in opposed position to said specimenengaging means, a straight resilient member extending from the other end of each clip arm, a lever pivotally mounted on said frame between said clip arms, one end of said lever engaging said straight resilient members, a hooked member on the other end of said lever, a trigger mem- -ber mounted on said movable arm, said trigger member adapted to slidably engage said hooked member, a resilient member attached between said frame and the end of the lever engaging said straight resilient members, and an elongated flexible member mounted at its extremities to the ends of the arms opposite :the specimenengaging means.

4. A gage comprising a frame, a xed arm secured to said frame and extending therefrom, a movable arm loosely mounted in said frame, said arms and frame forming a substantially U-shaped unit, specimen-engaging means mounted on the end of each arm, a clip arm pivotally mounted on each of said arms and having one end in opposed position to said specimenengaging members, a straight resilient member extending from the other end of each clip arm, a lever pivotally mounted on said frame between said clip arms, one end of said lever engaging said straight resilient members, a hooked member on the other end of said lever, a trigger pin mounted on said movable arm, said trigger pin adapted to slidably engage said hooked member, a resilient member attached between said frame and the end of the lever engaging said straight resilient members, an elongated flexible member mounted at its extremities to the ends of said arms opposite the specimen-engaging means, and means to indicate the degree of ilexing of said flexible member.

5. A gage comprising a frame, a xed arm secured to said frame and extending therefrom, a movable arm loosely mounted in said frame, said arms and frame forming a substantially U-shaped unit, specimen-engaging means mounted on the end of each arm, a clip arm pivotally mounted on each of said arms, and having one end in opposed position to said specimenengaging means, a straight resilient member extending from the other end of each clip arm, a lever pivotally mounted, on said frame between said clip arms, o-ne end of said lever engaging said straight resilient member, a hooked mem-l ber on the other end of said lever, a trigger pin mounted on said m-ovable arm, said trigger pin adapted to slidably engage said hooked member, a resilient member attached between said frame and the end of the lever engaging said straight resilient members, an elongated flexible member having one extremity xedly mounted in the frame adjacent the fixed arm, and mounted at the other extremity to the end of the movable arm opposite said specimen-'engaging means, and means mounted on said flexible member to indicate the degree of flexing.

6. A gage comprising a frame, a fixed arm secured to said frame and extending therefrom, a movable arm loosely mounted in Said frame, said arms and frame forming a substantially U-shaped unit, specimen-engaging means mounted on the end of each arm, a rst upright support mounted on each arm, a clip arm pivotally mounted on each upright support and having 4one end opposing said specimen-engaging means, a straight resilient member extending from the other end of each clip arm, a second upright support mounted on said frame lbetween said clip arms, a lever pivotally mounted in said second support, one end of said lever engaging said straight resilient members, a hooked member on the other end of said lever, a trigger pin mounted on said movable arm, said trigger pin adapted to slidably engage said hooked member, a resilient member attached between said frame and the end of the lever engaging said straight resilient members, an elongated iiexible member having one extremity fixedly mounted in the frame adjacent the lxed arm and mounted at the other extremity to the end of the movable arm opposite the specimen-engaging means, and means mounted on said exible member to indicate degree o-f flexing.

JACK REBMAN.

REFERENCES CITED The following references are of record in the iile of this patent:

UNITED STATES PATENTS Great Britain July 26, 1946 

